1. Field of the Invention
The present invention relates to methods and apparatus for increasing the contrast between the light scattering and non-light scattering regions in liquid crystal light valves and more particularly in liquid crystal light valves where the scattering is of the "emulsion scattering storage" (ESS) type.
2. Description of the Prior Art
Liquid crystal light valves whose light transmissivity in selected areas can be varied in response to a voltage, current, electric field, magnetic field, electron beam, a light beam or other external stimuli are well known. Such devices generally comprise a thin liquid crystal film or layer sandwiched between two plates at least one of which is transparent. A means is then provided for causing an image to be impressed on the liquid crystal by stimulating the liquid crystal film into its non-normal state. Generally, the normal state of a liquid crystal film is characterized by an ordering of the liquid crystal molecules in layers, columns or both layers and columns depending on whether the liquid crystal compound is of the smectic, nematic or cholesteric type respectively. Such an ordered state of the liquid crystal film is generally transparent. When the imaging stimulus is applied to the liquid crystal film, the molecules are induced into a non-ordered or "scattered" state. Since the regions where scattering occurs are generally translucent, an image can be impressed by the stimulation of selected regions of the liquid crystal cell.
Many liquid crystal materials are available for use in liquid crystal light valve devices. Devices using nematic type liquid crystals, for example, generally sustain light scattering only while the external stimulus is being applied, returning to the normal state within a relatively short time after the stimulus is removed. Scattering of this type is called "dynamic scattering."
In other types of liquid crystal materials, for example a nematic liquid crystal doped with about 10% cholesteric type liquid crystal, spontaneous return to the normal state either does not occur at all or occurs very slowly thus allowing retention of an image for a long period of time after the stimulus is removed.
Thus, it's clear that in storage-type liquid crystals not only must a stimulus be applied to cause a transition from a non-scattering to a scattering state, but an additional stimulus must be applied to cause the reverse transition from the scattering to the non-scattering state. The scattering in this type of liquid crystal device is called "emulsion scattering storage." It is this type of scattering to which the present invention is particularly applicable.
The method of stimulating storage-type liquid to crystals into emulsion scattering storage (ESS) by the application of an ac or dc field across the liquid crystal film is well known. The characteristics of the field required to induce transitions between the scattered and non-scattered state in storage-type liquid crystals are dependent upon the composition of the liquid crystal material and the geometric characteristic of the device. Thus, the application of a voltage across the cell having both a voltage amplitude above an amplitude threshold and a frequency below a maximum scattering frequency will cause a transition from the non-scattering to the scattering state. On the other hand, the application of a voltage across the cell having both a voltage amplitude above the amplitude threshold and a frequency greater than the maximum scattering frequency will cause erasing to begin to occur (i.e., transition from the scattered to the non-scattered state). Thus, in the present application the amplitude threshold refers to the dc or rms ac signal amplitude required to cause a transition from one scattering state to another, i.e., either to store an image or to erase a stored image. Further, the maximum scattering frequency the highest frequency at which scattering will occur in the liquid crystal if the voltage amplitude is above the amplitude threshold. A frequency below the maximum scattering frequency will cause scattering to occur while a frequency above the maximum scattering frequency will tend to cause erasure, i.e., restoration to the transparent state of the liquid crystal.
A representative imaging technique for transforming regions of the liquid crystal from the non-scattered to the scattered state utilizing such an ac signal is disclosed in U.S. Pat. No. 3,680,950. By contrast, in U.S. Pat. No. 3,642,348, a technique is disclosed which may be used to erase a stored image whereby transformation of specific portions of the liquid crystal from the Grandjean or scattered state to the focal-conic or non-scattered state is produced by the application of a second electric field.
Despite this ability to cause transitions between the scattered and non-scattered state in storage-type liquid crystals, a persistent problem has been the lack of sufficient contrast between the scattered and non-scattered regions resulting in difficulty in observing the image impressed on the liquid crystal film. Various techniques for increasing the contrast ratio between the scattered and non-scattered regions of a liquid crystal light valve have thus been devised. In U.S. Pat. No. 3,947,183, for example, a method of imaging is disclosed wherein increased contrast between the scattered and non-scattered regions is achieved by applying a scattering potential to the region desired to be scattered and applying an aligning electric field to the remainder of the liquid crystal area. However, this method requires complex addressing electronics to apply the appropriate electric field in the selected areas of the liquid crystal. In addition, the electric field is of the same strength as the imaging field. Another method of increasing contrast between scattered and non-scattered regions is disclosed in U.S. Pat. No. 3,652,148 wherein external means such as polarizers, edge lighting or optical filter systems are provided. Such means would require additional optical apparatus which would be cumbersome or otherwise unsuitable in many applications.
Thus, it is the general object of the present invention to provide a method by which the contrast between the scattering and non-scattering portions of a liquid crystal light valve is increased.
It is a further object of the present invention to provide a simplified, low power, method of increasing the contrast ratio in emulsive scattering storage liquid crystals.
It is yet another object of the present invention to provide a method to increase the contrast ratio without causing the image to be erased.
It is still another object of the present invention to provide an improved storage type liquid crystal device wherein a means is provided to increase the contrast ratio between the scattering and non-scattering regions.